Remove webhooks for anything but alerts

[ChrisSchinnerl]

With v2 we can get rid of webhooks for cache invalidation. I had a discussion with @n8maninger and we came to the conclusion that it's fine to only rely on short (e.g. 5 minute) caching for contracts/hosts.

Gouging settings should be polled every time they are needed. However, we only need them on upload within the worker. For downloads we can consider them good enough at the time we fund the account, which we do in the bus now.